It is now clearly established that the extent and duration of the effect of administered curare are fairly widely variable from one patient to another.
If it is then desirable to inject in the patient the amount of curare necessary for carrying out the surgical act and which is at the same time the lowest possible, it is necessary to be able to measure the degree of curarization.
Furthermore, safety of awakening depends on the most accurate evaluation possible of the decurarization (so of the residual curarization) which it is just as desirable to try and quantify whenever possible.
Quantification of curarization may be obtained by checking the secondary muscular response to stimulation of the motor nerve.;
Up to present, apparatus are known forn providing such checking (FR-A-1 359 777).
Such an apparatus comprises mainly:
an electric stimulator itself including a pulse generator connected to electrodes applied to the patient at the level of one of the passage points of the chosen motor nerve,
a sensor detecting the muscular response to be monitored with means for adapting it to the patient,
an analyzer analysing the signal emitted by the sensor.
The stimulator and the analyzer are obviously preferably designed so as to be able respectively to deliver different stimulations and to analyse the responses to these different simulations.
For electric stimulation, one of the cubital nerves is generally influenced and the transmission of the electric influx is ensured by members provided for this purpose such as subcutaneous electrodes or needles, which, connected to the apparatus, are disposed for example at the level of one of the wrists of said patient.
For detecting and measuring the muscular response in presently known apparatus, the sensor is disposed so as to detect and measure the adduction force at the level of the last phalanx of the thumb of the member at the level of which the stimulation is organized.
To correctly organize detection at the level of the hand concerned of the patient, it is necessary to immobilize this hand and to preposition the sensor.
For this, a device is known (US-A-4 236 528) which immobilizes one of the forearms of the patient in a position with the hand flat.
Other than a support plate against which the hand is placed, the device includes means for fixing the forearm and the hand on the plate, such as straps, and the detection member associated with the plate by a means for adjusting its position therefore allowing the device to be adapted to the dimensions of the patient's hand.
Besides it traumatic aspect for the patient and its considerable size, it is blamed for forcing the adductor of the last phalanx of the thumb to work in a plane more particularly defined by the support plate and in all cases appreciably distant from the natural adduction plane of this member which adversely affects the adduction reactions thereof.
In another device, the sensor consists of a piston sliding axially in a cylindrical body clamped in the hand which is held closed by straps and acting on a stress gauge.
Besides the problem of the wrong position involved, this apparatus also requires the piston to be acted on by the last phalanx of the thumb, therefore the reproach made to these two devices is a lack of reliability due to the numerous parasite movements of the last phalanx of the thumb and to the parasites in the environment of the patient such for example as the action of an electric surgical knife or mechanical shocks.
The signals picked up and analyzed in presently known apparatus are then, for a large part, formed of parasite signals among which it is sometimes difficult to recognize those corresponding to the expected muscular responses, which may even be drowned in the wave of parasite signals which, like the others, are in the form of peaks which are more or less pronounced, whence an extremely considerable risk of errors in the results of the analysis.
To limit the effects thereof, a device is known (medical and biological engineering and computing vol. 23 No. 6 of Nov. 1985 pps 547-555 IFMBE London, GB. H. S Bradlow et al "Microcomputer Based Muscle Relaxation Monitor and Control for Clinical Use") whose analysing circuit and stimulator are connected up so as to read the signal picked up only at the moment when it is expected.
Although, in the meantime, the wrong interpretation of simple parasites is avoided, during reading, the parasites still exist and falsify such reading.
A device is also known (US-A-4 387 723) in which a frequency band of about 10 Hz of the signals received is systematically eliminated, which eliminates the corresponding parasites but does not take into account the developmenet of noise and either parasites continue to exist or the signals picked up are amputated for no reason.
Moreover, if it were only for a greater linearity of the response detected, instead of being simply applied against the member where the response is expected, the piston of the sensor is generally applied against said member with a certain force called "preload" which depends on different factors such as the sensitivity of the patient to pulses.;
Unfortunately, these factors may themselves evolve during the time that the analysis lasts and falsify the result.